1. Field of the Invention
The present invention relates to a dielectric resonator apparatus and a high-frequency band-pass filter apparatus utilizing the dielectric resonator apparatuses, and in particular, to a TM dual mode dielectric resonator apparatus and a high-frequency band-pass filter apparatus utilizing the TM dual mode dielectric resonator apparatuses.
2. Description of the Related Art
FIG. 9 is a partially broken-away perspective view of a conventional high frequency four-stage band-pass filter apparatus 51, which comprises two single mode dielectric resonator 52 and 53 and one TM dual mode dielectric resonator 54 which are provided within a metal case 55.
Referring to FIG. 9, each of the single mode dielectric resonators 52 and 53 is constituted by providing a TM mode dielectric resonator 57 provided within an electrically conductive case 56 which functions as a waveguide. Further, the TM dual mode dielectric resonator 54 is constituted by providing within an electrically conductive case 61, a TM dual mode dielectric resonator 60 integrally formed in a shape of a cross of two TM mode dielectric resonators 58 and 59 so that the TM mode dielectric resonators 58 and 59 are perpendicular to each other, and the TM dual mode dielectric resonator 60 has coupling grooves 70 for coupling an operation mode of the even mode with that of the odd mode. The TM dual mode dielectric resonator 54 is disclosed in, for example, the Japanese Patent Laid-open Publication No. 61-121502.
In the conventional band-pass filter apparatus 51, a coupling loop 63, which is provided on the side of the inner surface of a coaxial input and output connector 62 provided on one end surface of a metal case 64 and which is electrically connected to the coaxial input and output connector 62, is magnetically coupled with the TM mode dielectric resonator (not shown) of the single mode dielectric resonator apparatus 52 of the first stage, which is magnetically coupled with the TM dual mode dielectric resonator 60, which provides the second and third stages. Further, the TM dual mode dielectric resonator 60 is magnetically coupled with the TM mode dielectric resonator 57 of the single mode dielectric resonator apparatus 53 of the final stage, which is magnetically coupled with another coupling loop (not shown) electrically connected to another coaxial input and output connector (not shown). This results in the four-stage band-pass filter apparatus 51.
By using a TM dual mode dielectric resonator, since the two TM mode dielectric resonators therein are integrally formed in a form of a cross, it is considered that the size of the filter apparatus can be reduced as compared with a case in which there are provided two single mode dielectric resonators. Therefore, in the above-mentioned four-stage band-pass filter apparatus, if two TM dual mode dielectric resonators can be utilized, rather than only one, it is expected that the size of the bandpass filter apparatus can be further reduced.
However, in the conventional TM dual mode dielectric resonator, the two TM mode dielectric resonators thereof have the same resonance frequency before they are installed within the filter apparatus. But when the TM dual mode dielectric resonator has been installed within the above-mentioned filter apparatus, and the TM dual mode dielectric resonator is coupled with the coupling loop, the resonance frequency of the one TM mode dielectric resonator thereof magnetically coupled with the coupling loop becomes different from that of the other TM mode dielectric resonator thereof not coupled with the coupling loop, due to the influence of the coupling loop. When the two TM mode dielectric resonators of the TM dual mode dielectric resonator have different resonance frequencies from each other, the coupling coefficient between the operation modes of the two rectangular-parallelepiped-shaped TM mode dielectric resonators of the TM dual mode dielectric resonator cannot be determined on the basis of the resonance frequency f.sub.even of the even mode and the resonance frequency f.sub.odd of the odd mode.
Accordingly, in the conventional apparatuses, as shown in the filter apparatus of FIG. 9, it is conventional to have single mode dielectric resonators 52 and 53 which are magnetically coupled with the coupling loops 63. In this case, the problem remains that the size of the conventional filter apparatus can not be further reduced.